This invention relates to a communications apparatus, communications control method, system and storage medium for transmitting data from a transmitting terminal to a receiving terminal via a network. More particularly, the invention relates to real-time transfer of data.
In systems that are now available, a connection is made to a communications network such as the Internet and real-time data such as motion pictures and sound obtained by a video camera and microphone ancillary thereto is transmitted through the network. When such a system is constructed, it is necessary to cope with the conditions of communications lines the states of which are constantly changing. More specifically, it is necessary to send data from the transmitting side at a rate at which it can be received on the receiving side.
In order to control the rate at which data is sent on the transmitting side, it is required to know the effective receiving rate on the receiving side. To achieve this, not only must the original data transmission be performed but it is also necessary for the transmitting and receiving sides to exchange control information.
RFC 1889, which is known as a Real-Time Transport Protocol (RTP), can be mentioned as such a communications protocol. With the Real-Time Transport Protocol, RTP (RTP Data Transfer Protocol) is used to send and receive data, and RTCP (RTP Control Protocol) is used to send and receive control information.
In the prior art, however, the sending and receiving of control information is carried out periodically (once every five seconds is recommended as the transmission interval of RTCP according to RTP). When the conditions on a communications line deteriorate, therefore, there is a delay in the detection of the change and a delay in feedback information indicating that the transmission rate was changed to cope with the altered conditions.
For example, in a system in which there is a flow of control information once every five seconds, assume that a change in line conditions is detected five seconds after the change in a worst-case scenario and that the transmission rate on the transmitting side is altered as a result. Whether the altered transmission rate is appropriate or not is then judged. This judgment cannot be made until after ten seconds in the worst case. When line status changes, therefore, there is an increase in dropped packets and a delay in packet distribution occurs.
Further, when real-time data is transmitted at a transmission rate that exceeds the bandwidth of the communications line, (1) the transmitted data is stored in the buffer of a network router or the like and, hence, there is an increase in the transmission delay, and (2) if the buffer subsequently overflows, packets are lost.
In implementations heretofore available, packet loss is monitored and transmission rate is reduced when packet loss occurs. Transmission rate thus is controlled in such a manner that packet loss will not occur.
With these conventional implementations, however, the aforementioned problem (1) of transmission delay cannot be solved even if problem (2) can be. Consequently, when these implementations are used to transmit video, as in a videoconference or the like, images that are eventually displayed on the receiving side were actually captured several tens of seconds earlier. Thus, a problem with the prior art is a long transmission delay.
Accordingly, an object of the present invention is to provide a communications control method, system and storage medium that make it possible to transmit data efficiently by transmitting control information in dependence upon the status of a network.
Another object of the present invention is to provide a communications apparatus, method, system and storage medium that make it possible to communicate with the minimum transmission delay by establishing the proper transmission rate dynamically in dependence upon the status of a network.
According to the present invention, the foregoing objects are attained by providing a communications control method for transmitting data from a transmitting terminal to a receiving terminal via a prescribed network and transmitting, at prescribed intervals, control information for ascertaining the status of the network, the method comprising the steps of detecting a change in the status of the network, and adjusting, in dependence upon the detected change in status, the frequency with which the control information is transmitted.
Further, according to the present invention, the foregoing objects are attained by providing communications apparatus for transmitting information to a receiving terminal via a prescribed network and receiving reception-status information transmitted by the receiving terminal at prescribed time intervals, comprising detecting means for detecting state of a transmission delay based upon a plurality of items of reception-status information from the receiving terminal, and control means for controlling transmission rate in dependence the detected state of the transmission delay.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.